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Components in internal combustion engines are often subjected to temperature cycling that lead to low-cycle
fatigue due to thermal expansion and contraction. Furthermore, these thermal loads may be multiaxial and
non-proportional in nature which means that traditional fatigue evaluation methods are not sucient. Welds
constitute another complication since they are often more sensitive to fatigue damage than the base material.
This is in large part due to the notch eect associated with the boundaries of the weld in addition to the
presence of tensile residual stresses near the weld.
At Volvo Cars, there is an interest in developing a new methodology for numerically evaluating the low
cycle fatigue life of welds during thermal cycling. Methods that can account for non-proportional loading are of
particular interest. One of the most prevalent methods for non-proportional multiaxial fatigue evaluation is the
critical plane approach. Evaluating weld fatigue also requires treatment of the weld geometry to resolve the
stress-strain gradients in the vicinity of the weld. This thesis explores the possibility of combining the critical
plane approach with common weld modelling techniques to accurately model low cycle thermal fatigue. The
Smith-Watson-Topper model using the maximum normal stress to the critical plane was chosen as the critical
plane fatigue model and compared to a traditional strain-based fatigue evaluation methodology. Welds were
modelled with shell elements, solid elements and with the eective notch method. Weld residual stresses were
accounted for by considering them as a tensile mean stress combined with a mean stress correction. To assess
and compare the methods, a welded exhaust manifold subjected to low cycle thermal fatigue was evaluated by
using Finite Element Analysis (FEA).
It was shown that both the critical plane approach and traditional strain-based fatigue evaluation oered a
conservative fatigue life estimate compared to available experimental values. It was also shown that modelling
the weld with shell elements resulted in fatigue life estimates within the margin of error for the experimental
values. Using the eective notch method gave the lowest fatigue life estimate but the fatigue failure location
was predicted to be the weld toe which corresponded with known failure locations from experimental testing.
Mean stress eects were shown to have a negligible impact at the considered fatigue lives. It is important to
note that further numerical and experimental validation is needed before deploying the methodology in an
industrial setting.

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BibTeX @mastersthesis{Alm2018,author={Alm, Jakob},title={Critical plane approach to low cycle thermal fatigue of welds in exhaust manifolds},abstract={Components in internal combustion engines are often subjected to temperature cycling that lead to low-cycle
fatigue due to thermal expansion and contraction. Furthermore, these thermal loads may be multiaxial and
non-proportional in nature which means that traditional fatigue evaluation methods are not sucient. Welds
constitute another complication since they are often more sensitive to fatigue damage than the base material.
This is in large part due to the notch eect associated with the boundaries of the weld in addition to the
presence of tensile residual stresses near the weld.
At Volvo Cars, there is an interest in developing a new methodology for numerically evaluating the low
cycle fatigue life of welds during thermal cycling. Methods that can account for non-proportional loading are of
particular interest. One of the most prevalent methods for non-proportional multiaxial fatigue evaluation is the
critical plane approach. Evaluating weld fatigue also requires treatment of the weld geometry to resolve the
stress-strain gradients in the vicinity of the weld. This thesis explores the possibility of combining the critical
plane approach with common weld modelling techniques to accurately model low cycle thermal fatigue. The
Smith-Watson-Topper model using the maximum normal stress to the critical plane was chosen as the critical
plane fatigue model and compared to a traditional strain-based fatigue evaluation methodology. Welds were
modelled with shell elements, solid elements and with the eective notch method. Weld residual stresses were
accounted for by considering them as a tensile mean stress combined with a mean stress correction. To assess
and compare the methods, a welded exhaust manifold subjected to low cycle thermal fatigue was evaluated by
using Finite Element Analysis (FEA).
It was shown that both the critical plane approach and traditional strain-based fatigue evaluation oered a
conservative fatigue life estimate compared to available experimental values. It was also shown that modelling
the weld with shell elements resulted in fatigue life estimates within the margin of error for the experimental
values. Using the eective notch method gave the lowest fatigue life estimate but the fatigue failure location
was predicted to be the weld toe which corresponded with known failure locations from experimental testing.
Mean stress eects were shown to have a negligible impact at the considered fatigue lives. It is important to
note that further numerical and experimental validation is needed before deploying the methodology in an
industrial setting.},publisher={Institutionen för industri- och materialvetenskap, Chalmers tekniska högskola},place={Göteborg},year={2018},keywords={Weld fatigue; Critical plane approach; Exhaust manifold; Low cycle fatigue; Thermal fatigue;},note={34},}

RefWorks RT GenericSR ElectronicID 255473A1 Alm, JakobT1 Critical plane approach to low cycle thermal fatigue of welds in exhaust manifoldsYR 2018AB Components in internal combustion engines are often subjected to temperature cycling that lead to low-cycle
fatigue due to thermal expansion and contraction. Furthermore, these thermal loads may be multiaxial and
non-proportional in nature which means that traditional fatigue evaluation methods are not sucient. Welds
constitute another complication since they are often more sensitive to fatigue damage than the base material.
This is in large part due to the notch eect associated with the boundaries of the weld in addition to the
presence of tensile residual stresses near the weld.
At Volvo Cars, there is an interest in developing a new methodology for numerically evaluating the low
cycle fatigue life of welds during thermal cycling. Methods that can account for non-proportional loading are of
particular interest. One of the most prevalent methods for non-proportional multiaxial fatigue evaluation is the
critical plane approach. Evaluating weld fatigue also requires treatment of the weld geometry to resolve the
stress-strain gradients in the vicinity of the weld. This thesis explores the possibility of combining the critical
plane approach with common weld modelling techniques to accurately model low cycle thermal fatigue. The
Smith-Watson-Topper model using the maximum normal stress to the critical plane was chosen as the critical
plane fatigue model and compared to a traditional strain-based fatigue evaluation methodology. Welds were
modelled with shell elements, solid elements and with the eective notch method. Weld residual stresses were
accounted for by considering them as a tensile mean stress combined with a mean stress correction. To assess
and compare the methods, a welded exhaust manifold subjected to low cycle thermal fatigue was evaluated by
using Finite Element Analysis (FEA).
It was shown that both the critical plane approach and traditional strain-based fatigue evaluation oered a
conservative fatigue life estimate compared to available experimental values. It was also shown that modelling
the weld with shell elements resulted in fatigue life estimates within the margin of error for the experimental
values. Using the eective notch method gave the lowest fatigue life estimate but the fatigue failure location
was predicted to be the weld toe which corresponded with known failure locations from experimental testing.
Mean stress eects were shown to have a negligible impact at the considered fatigue lives. It is important to
note that further numerical and experimental validation is needed before deploying the methodology in an
industrial setting.PB Institutionen för industri- och materialvetenskap, Chalmers tekniska högskola,LA engLK http://publications.lib.chalmers.se/records/fulltext/255473/255473.pdfOL 30